In recent years, thin batteries have been in increasing demand with the spread of portable devices, such as video cameras and laptop computers. A typical thin battery is a lithium ion polymer secondary battery which is formed by laminating a positive electrode and a negative electrode. The positive electrode is made by forming a positive electrode active material layer on the surface of a sheet-like positive electrode current collector, and the negative electrode is made by forming a negative electrode active material layer on the surface of a sheet-like negative electrode current collector. An electrolyte layer is interposed between the positive electrode active material layer and the negative electrode active material layer. In this battery, a positive electrode terminal and a negative electrode terminal are provided on the positive electrode current collector and the negative electrode current collector, respectively, for leading out a current generated by a potential difference between the two active materials, and the laminate formed thereby is hermetically sealed in a package so as to form the lithium ion polymer secondary battery. The lithium ion polymer secondary battery uses the positive electrode terminal and the negative electrode terminal which lead out of the package as the battery terminals to provide predetermined electrical output.
The lithium ion polymer secondary battery having such a structure has high battery voltage and high energy density, and it is viewed as very promising. Bonding layers are often interposed between the current collectors and the active material layers. Characteristics which the bonding layer is required to have include sufficient force of adhesion with the current collector material, sufficient force of bonding with the binder contained in the active material layer, stability in the presence of an organic solvent in the electrolytic solution, excellent long-term storage stability, thermal stability to remain without peeling when exposed to high temperature and electrochemical stability to endure repetitive charge-discharge cycles, but there has not been a solution that meets these requirements.
For example, it is necessary to increase the surface areas of the positive electrode sheet and the negative electrode sheet in order to further increase the discharge capacity of the lithium ion polymer secondary battery. However, simply increasing the surface areas of the positive electrode sheet and the negative electrode sheet has a drawback of making the electrodes difficult to handle because of the large surface areas. Solutions may be conceived for this problem, such as folding or winding the large positive electrode sheet and negative electrode sheet to a predetermined size. When the positive electrode sheet or the negative electrode sheet are folded or wound in a laminated state, the positive electrode sheet or the negative electrode sheet is bent along the folding line, thus causing the positive electrode sheet or the negative electrode sheet to peel off from the electrolyte layer, such as polymer electrolyte layer, which leads to a decrease in the effective surface area of the interface between the electrode and the electrolyte, resulting in decreasing charge capacity and internal resistance of the battery, thus deteriorating the cycle characteristics of the charge capacity. Also, there has been a problem in that the current collector peels off the active material layer formed on the positive electrode sheet and the negative electrode sheet due to bending along the folding line. Moreover, there has been a problem in that the positive and negative electrode active material layers expand and contract as the positive and negative electrode active material layers store and release lithium ions in the charge and discharge cycles, resulting in a stress that causes the active material layer to peel off the current collector. To solve the problems described above, a method has been proposed such as a bonding layer being formed between the active material layer and the current collector so as to prevent the layers from peeling off and adhesion from being reduced by means of the bonding layer.
The bonding layers, that are interposed between the positive electrode active material layer and the positive electrode current collector and between the negative electrode active material layer and the negative electrode current collector, are required to have both the function of holding the two members together and the function of providing electrical conductivity, and are therefore formed by dispersing an conductive substance in a polymer material used as a binder that holds the two members together.
Prior art techniques that aim to solve the problems described above include one in which a bonding layer is interposed between the active material layer and the current collector so as to prevent the layers from peeling off and adhesion from being reduced by means of the bonding layer, as disclosed in the prior art documents (1) to (5) as follows: (1) Japanese Examined Patent Publication No. 7-70328 discloses a current collector coated with a conductive film mentions of a binder and a conductive filler. This invention names phenol resin, melamine resin, urea resin, vinyl resin, alkyd resin, synthetic rubber and the like as the binder material. (2) Japanese Unexamined Patent Publication No. 9-35707 discloses a constitution in which a negative electrode material layer containing a binder made of powdered carbon and polyvinylidene fluoride (hereinafter referred to as PVdF) is formed on the negative electrode current collector, and a binder layer made of an acrylic copolymer containing a conductive substance mixed therein is formed on the negative electrode current collector. This invention achieves a high bonding effect by using the acrylic copolymer having high strength of bonding with copper for the negative electrode plate whereon the negative electrode current collector is formed from a copper foil. (3) Japanese Unexamined Patent Publication No. 10-149810 discloses a constitution in which an undercoat layer is formed by applying a polyurethane resin or an epoxy resin between the active material layer and the current collector. This invention improves adhesion between the active material layer and the current collector in the electrode by forming the undercoat layer from polyurethane resin or epoxy resin, thereby improving the cycle capacity maintaining characteristics of the battery.
(4) Japanese Unexamined Patent Publication No. 10-144298 discloses a constitution as a bonding layer made of graphite and a binder between the negative electrode current collector and the negative electrode active material layer. According to this invention, graphite contained in the bonding layer improves the efficiency of the negative electrode in accumulating charge. (5) Japanese Unexamined Patent Publication No. 9-213370 discloses a constitution in which graft-polymerized PVdF is used as a binder for the electrolyte portion and the electrolyte layer of the battery active material. This invention improves the efficiency of making contact with the current collector by using graft-polymerized PVdF as a binder for the electrolyte portion and the electrolyte layer of the battery active material.
Characteristics which the bonding layer is required to have include sufficient force of adhesion with the current collector material, sufficient force of bonding with the binder contained in the active material layer, stability in the presence of an organic solvent in the electrolytic solution, excellent long-term storage stability, thermal stability to remain without peeling when exposed to high temperature and electrochemical stability to endure repeated charge-discharge cycles.
However, the technique (1) has a problem in that butyl rubber, phenol resin or the like used as the binder is corroded with the electrolytic solution and peels off. In the technique (2), although adhesive strength between the negative electrode current collector and the negative electrode material layer can be increased by forming the bonding layer containing, as a major component, acrylic copolymer containing a conductive substance mixed therein between the negative electrode current collector and the negative electrode material layer because an acrylic copolymer has high strength of bonding with PVdF contained in the negative electrode material layer and the negative electrode current collector, there has been a problem in that the acrylic copolymer is corroded by the electrolytic solution and the negative electrode current collector peels off the negative electrode material. Although the technique (3) is claimed to increase the peel-off resistance and the number of 80% capacity cycles in the case in which polyurethane resin is used as the undercoat layer, the effect is not practically sufficient. When an epoxy resin is used, the epoxy resin is corroded by the electrolytic solution, and therefore there is a possibility that the active material layer will peel off the current collector.
In the technique (4), although satisfactory adhesive strength between the bonding layer and the active material is achieved since the bonding layer includes a material similar to the binder contained in the active layer, the strength of bonding with the current collector is not satisfactory, and it is comparable to that in a case in which the active material layer is formed directly on the current collector. Also, because the electrolytic solution infiltrates the binder, there is a problem of weak bonding strength between the bonding layer and the current collector. In the technique (5), the active material layer can be formed directly on the current collector without using a bonding layer since the graft-polymerized material that has high strength of bonding with the current collector is used as the binder of the active material layer, but it has a drawback in that solvents that can be used are limited since the polymer is difficult to dissolve. Also, because it is difficult to completely remove the solvent from the inside of the battery, there is a possibility that the solvent remaining in the battery will cause an adverse effect on the battery performance.
In the second prior art technique to solve the problems described above, powdered carbon is dispersed as a conductive material in the bonding layer. However, the powdered carbon does not provide sufficient electrical conductivity, and it is necessary to increase the weight ratio of the powdered carbon to the binder, (powdered carbon/binder), in order to obtain satisfactory electrical conductivity. When the proportion of the powdered carbon in the bonding layer is increased, the proportion of the binder in the bonding layer decreases and the contact area of the binder with the current collector and the active material layer decreases due to the large volume of the powdered carbon, thus resulting in insufficient adhesive strength.
As the third prior art technique to solve the problems described above, a battery electrode is disclosed in which a bonding layer is formed in a pattern of dots, stripes, or a grid between the current collector and the active material layer (Japanese Unexamined Patent Publication No. 11-73947). The battery electrode is provided with a paint to form the bonding layer applied thereto by spraying or printing. The area in which the paint of the bonding layer is applied is in a range from 30 to 80% of the active material holding area of the current collector.
In the battery electrode constituted as described above, since the bonding layer is formed in a predetermined pattern between the current collector and the active material layer, adhesion between the two members can be improved without impeding the exchange of electrons between the current collector and the active material layer, thereby improving the cycle characteristics. Specifically, adhesion between the current collector and the active material layer is maintained by the bonding layer that has the predetermined coating pattern, so that the exchange of electrons between the current collector and the active material layer is carried out smoothly in portions that are not coated, thereby keeping the electrical resistance low.
As another technique that belongs to the third prior art technique described above, a battery electrode is disclosed in which a binder that constitutes the electrode is uniformly dispersed in the electrode material (Japanese Unexamined Patent Publication No. 7-6752). This electrode is produced by forming an electrode containing a binder material dispersed therein on a current collector and drying the electrode, followed by pressure forming and further heat treatment.
A high performance secondary battery having excellent charge capacity characteristics, particularly cycle characteristics, can be produced by using the electrode that has the constitution specified in the two documents as described above.
However, among the third prior art techniques described above, the battery electrode of the prior art described in Japanese Unexamined Patent Publication No. 11-73947 requires it to form the bonding layer in a pattern of dots, stripes, or a grid, and there is a problem in that it is very difficult to form the bonding layer. Also, there remains a problem in that, in the case in which the area of portions which are not coated, where electrons are exchanged between the current collector and the active material layer, is relatively large, sufficient adhesion cannot be maintained in the portions and peel-off occurs.
In the electrode of the prior art disclosed in Japanese Unexamined Patent Publication No. 7-6752, the polymer used as the binder or the polymer electrolyte that provides binding effect is completely dissolved in the solvent, and this is mixed uniformly with other materials such as carbon and the active material, thereby preparing a coating slurry. As a result, sufficient adhesive strength cannot be achieved between the current collector and the active material layer, thus the problem of decreasing charge/discharge cycle characteristics of the battery remains to be solved. This problem is supposed to be due to the presence of a large quantity of powdered material, such as carbon, that is added to the coating slurry, in the interface between the current collector and the active material layer.